The Mysterious Source of Extragalactic Cosmic Rays

The particles that shouldn’t exist

An artist rendering shows particles entering the atmosphere where they spark air showers that can spread for miles. Image by NASA.

It’s the year 1912 and the idea of the American Dream is giving hope to US citizens and immigrants alike. Farmers begin to trickle into the booming city and accents mingle in populated areas like New York where Italians work in the coal mines and Russian and Polish couples front the pushcart markets of the busy streets. Stories like those of Andrew Carnegie or John D. Rockefeller give people hope that they too, someday, can make great incomes and establish families in the land of opportunity. Low wages and dangerous working conditions give rise to the first labor unions and out at sea, a passenger liner by the name of the RMS Titanic sinks in the late night hours of April 14th.

This same year, scientists were working on experiments using electroscopes — instruments for detecting electric charge, or what could be thought of as early radiation detectors. The experiments revealed an ever-present ambient radiation that didn’t seem to be coming from any one particular place. The radiation was still there even when recreating the experiments underground. This lead to the hypothesis that it must be originating from the Earth itself, a sort of terrestrial signal. It came as a big surprise, then, when those same electroscopes were sent up on hot air balloons and the signals increased significantly after a small, initial drop. This initial drop was due to radiation interacting with air molecules and weakening before spiking again. The signal wasn’t terrestrial at all. They were cosmic in origin and, in fact, must be extra-galactic at the higher energy levels.

The aptly named “cosmic rays” have become one of the biggest mysteries in physics.

While some lower energy rays are the results of processes in the sun, it’s believed most of them originate from supernova explosions within the Milky Way. The exploding star will result in shock waves that then trap particles in a magnetic field. The particles become accelerated until they’ve gained enough energy to escape into space. These charged, energetic particles are what we then detect here on Earth and refer to as cosmic rays. The particles can range from positively charged protons to heavier ionized nuclei like oxygen and iron, though only about 1% of cosmic rays are made of heavier nuclei. Secondary cosmic rays like muons and electrons pass through you everyday, but the higher energy particles that shocked scientists upon their discovery are rare and much more difficult to detect. How they’re made and where they come from is a mystery with some paradoxical terms.

While supernova explosions can explain low energy cosmic rays, they aren’t capable on their own of accelerating particles to ultra-high levels. Image by NASA/JPL-Caltech.

In mid-October of 1991, The Oh-My-God particle was detected in Utah. It remains the highest energy particle ever detected. Before its discovery, scientists didn’t believe particles with such extreme energies could exist and yet this one contained millions of times more energy than anything possible at the Large Hadron Collider. Manmade particles can reach a maximum of about 10¹³eV (electronvolts) while the OMG particle reached energies of 3 x 10²⁰eV. Think of the energy of a baseball travelling over 50 mph (80 km/h) but packed into a single proton.

Cosmic rays of anything over 10¹⁸eV are categorized as ultra-high energy cosmic rays (UHECR) and are thought to come from outside of our galaxy. This is because higher energy particles are more difficult to constrain into smaller spaces — in this case, the small space is a galaxy. But their immense energy levels also mean they must come from grand and very violent events in the universe. Active galactic nuclei, gamma-ray bursts, pulsars and magnetars have all been speculated sources, though none of these are conclusive.

Research published last year attempts to link UHECR to starburst galaxies. Starburst galaxies are brilliant destinations in the universe where stars are made at a much higher rate than average. A series of supernova explosions in these galaxies can cause a huge amassing of outflowing gas, resulting in a shockwave that accelerates particles to near light speed. Magnetic fields trap particles where they attain more and more energy as they traverse the shock front. Eventually, they do gather enough energy to reach an escape velocity and go hurtling through space. However, according to the Pierre Auger Observatory, research pairing UHECR and starburst galaxies still leaves 90% of observations unaccounted for, meaning that a better match must still be out there.

The Pierre Auger Observatory in Argentina is home to 1,660 barrels of water that cover 1,860 m² (3,000 km²). Tubes mounted on the tanks respond to electromagnetic shockwaves when a particle enters a detector.

One of the problems with assuming that events like gamma-ray bursts and pulsars are the sources of these particles is that they couldn’t have come from far away. If the particles were any further out than 1–200,000,000 light years, all their energy would be lost through interactions with the cosmic background radiation. But events within that range should be clearly visible to us here on Earth, and yet no source for these rays has been found. Although, there does seem to be a hot spot around the Ursa Major (Big Dipper) cluster from where a quarter of these cosmic rays originate. The trouble with tracing these particles back to their sources is that, unlike neutrinos which travel in a straight line from source to destination, cosmic rays are deflected by magnetic fields during their travel (this includes the magnetic fields of our own Milky Way). If the particles are heavier, they’ll also change as they lose energy and break apart, morphing into a different particle type on their way to us.

Once arriving on our planet, detectors like CREAM (Cosmic-Ray Energetics and Mass investigation) situated in the stratosphere above Antartica can gather information about the energy, type, and direction of incoming particles. While the detector was installed on the space station (known as ISS-CREAM) where it was in better position to study UHECR, it was shut down in February of this year. Fortunately, arrays of detectors on the ground continue to gather data. When particles hit the atmosphere, they set off a cascade — an air shower — that excites nitrogen in the air. Once de-excited, radiation is produced that gives off a fluorescence detected by telescopes.

More exotic theories propose ultra-heavy dark matter that decays into the UHECR particles we see, though this idea suggests most particles should be photons and neutrinos. This isn’t the case according to current observations. Hypothetical objects left over from the Big Bang (domain walls or cosmic strings) could cause cosmic rays upon their collapse but no evidence for these structures has ever been found.

The red region in the Northern Hemisphere is where observed events happened the most frequently. This is a supposed “hot spot” of cosmic ray activity. Image by Telescope Array Collaboration.

Cosmic rays have a funny effect on people in space. They’re responsible for bright specks of light in astronauts’ eyes and can possibly lead to vision damage and cognitive side effects. The specks sometimes manifest in bolder streaks or clouds. Enough exposure can also lead to mutated cells, making cosmic rays not only compelling and mysterious, but one of the biggest obstacles to manned space travel. In many ways, we don’t know much more now than we did in the early 1900’s when these particles were first discovered. Throughout the years they have surprised us, eluded us, and now challenge us to innovate a way to live with them if we’re to travel through the universe as they do.